Influenza epidemics observed in primary care from 1984 to 2017 in France: A decrease in epidemic size over time.

Cécile Souty,Laëtitia Minodier,Ana‐Maria Vilcu,Yves Dorléans,Daniel Lévy‐Bruhl,Bruno Lina,Sylvie Van Der Werf,Thierry Blanchon,Alessandra Falchi,Caroline Guerrisi,Clément Turbelin,Shirley Masse,Lisandru Capai,Thomas Hanslik,Isabelle Bonmarin,Philippe Amoros,Louise Rossignol

doi:10.1111/irv.12620

Abstract

BackgroundEpidemiological analysis of past influenza epidemics remains essential to understand the evolution of the disease and optimize control and prevention strategies. Here, we aimed to use data collected by a primary care surveillance system over the last three decades to study trends in influenza epidemics and describe epidemic profiles according to circulating influenza viruses.MethodsInfluenza‐like illness (ILI) weekly incidences were estimated using cases reported by general practitioners participating in the French Sentinelles network, between 1984 and 2017. Influenza epidemics were detected by applying a periodic regression to this time series. Epidemic (co‐)dominant influenza virus (sub)types were determined using French virology data.ResultsDuring the study period, 297 607 ILI cases were reported allowing the detection of 33 influenza epidemics. On average, seasonal epidemics lasted 9 weeks and affected 4.1% of the population (95% CI 3.5; 4.7). Mean age of cases was 29 years. Epidemic size decreased over time by ‐66 cases per 100 000 population per season on average (95% CI −132; −0.2, P value = 0.049) and epidemic height decreased by ‐15 cases per 100 000 (95% CI −28; −2, P value = 0.022). Epidemic duration appeared stable over time. Epidemics were mostly dominated by A(H3N2) (n = 17, 52%), associated with larger epidemic size, higher epidemic peak and older age of cases.ConclusionsThe declining trend in influenza epidemic size and height over the last 33 years might be related to several factors like increased vaccine coverage, hygiene improvements or changing in influenza viruses. However, further researches are needed to assess the impact of potential contributing factors to adapt influenza plans.

Highlights

Influenza is a common respiratory infectious disease, manifested typically by influenza-like illness (ILI) usually defined by sudden onset of fever, myalgia and respiratory signs.[1]
The French Sentinelles network is a real- time epidemiologic surveillance system in primary care created in 1984.10 A sample of French general practitioners (GPs) located all over the country participates on a voluntary basis to data reporting
The stability of the methods and ILI definition used allowed the comparison of 32 seasonal influenza epidemics and a pandemic, observed between winters 1984/85 and 2016/17

Summary

Background

Epidemiological analysis of past influenza epidemics remains essential to understand the evolution of the disease and optimize control and prevention strategies. We aimed to use data collected by a primary care surveillance system over the last three decades to study trends in influenza epidemics and describe epidemic profiles according to circulating influenza viruses. Results: During the study period, 297 607 ILI cases were reported allowing the detection of 33 influenza epidemics. Epidemic size decreased over time by -66 cases per 100 000 population per season on average (95% CI −132; −0.2, P value = 0.049) and epidemic height. Epidemics were mostly dominated by A(H3N2) (n = 17, 52%), associated with larger epidemic size, higher epidemic peak and older age of cases. Conclusions: The declining trend in influenza epidemic size and height over the last 33 years might be related to several factors like increased vaccine coverage, hygiene improvements or changing in influenza viruses.

| INTRODUCTION

| METHODS

Findings

| DISCUSSION